[time-nuts] How does sawtooth compensation work?

Michael Gray mikenet213 at comcast.net
Tue Jul 19 12:32:43 EDT 2016


Alright, may I vote for tangent #3? I've heard this mentioned in passing
a few times on this list, but never seen it described in detail...so
much so that I have no idea what it means.

Are we talking an external, steerable LO/clock (similar to #2, except
for the location of the oscillator itself)? An external oscillator
triggering a timestamp against the GPS's internal clock (seems identical
to PPS error-wise, except you can introduce your own dither)? Something
else? Why does carrier phase tracking matter, as long as the GPS is
deriving a time solution through some mechanism that exceeds the
granularity of its CPU clock?

Forgive my ignorance here, but I'd love to see more of this tangent,
since I've never seen anyone jump into it in detail.

Michael

> There are some tangents we could go down:
> 1) There are cases where the inherent dithering you get from sawtooth error is actually hugely beneficial to the design of a GPSDO.
> 2) One GPSDO design (Trimble Thunderbolt) is unique in that is has no sawtooth problem or TIC or XO or TCXO at all. Instead it directly uses the high-quality OCXO as the receiver's LO. They get away with this clean solution because they are a company that makes their own receiver h/w.
> 3) Carrier phase receivers with external clock input.
>
> /tvb



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